1. Field of the Invention
The invention relates to a vinyl chloride resin composition with a flame retardancy, and an electric wire and a cable each covered by the resin composition.
2. Description of the Related Art
Wires/cables used inside electric/electronic devices or used for wiring between devices are required to be highly flame retardant (e.g., required to pass the VW-1 test of UL standard). Some cables have a structure in which a wire core or two or more wire cores each formed by covering a conductor with an insulation are covered with a metal shielding layer and a sheath (see FIGS. 1 and 2) and some of such cables or the like are also required to have high signal transmission characteristics. Insulation materials for such cables are desired to have a low permittivity and flammable resin compositions, such as polyethylene, which do not include an inorganic flame retardant are therefore used. Sheath materials of such cables are required to be more highly flame retardant in order to provide enough flame retardancy.
On the other hand, the wires/cables used inside electric/electronic devices or used for wiring between devices are often required to have flexibility so as to be flexibly wired and handled during use of devices. Flexibility of cable is greatly affected by flexibility of sheath material.
As such, for the wires/cables used inside electric/electronic devices or used for wiring between devices, the sheath material (or the insulation material for insulated wires) are required to have both high flame retardancy and flexibility.
Polyvinyl chloride resin compositions could be used as materials which satisfy such requirements. In general, polyvinyl chloride resin compositions are often used as a covering material of wires/cables because of having a favorable balance of mechanical characteristics, heat resistance, cold resistance, electrical insulation properties, flame retardancy, processability and economic efficiency which can be adjusted by changing a compounding ratio of plasticizer, stabilizer and flame retardant, etc. Flexibility of the polyvinyl chloride resin compositions is enhanced by increasing a compounding ratio of plasticizer, but flame retardancy of the composition decreases with increasing the compounding ratio of plasticizer since it is a flammable liquid. Therefore, polyvinyl chloride resin compositions including antimony trioxide as a flame retardant, alone or in conjunction with other flame retardants, have been used for covering materials of wires/cables (see JP-A-H07-149982). Compositions including a large amount of antimony trioxide are used especially when both high flame retardancy and flexibility are required as described above.
However, use of antimony compounds tends to be cut down in recent years due to concerns about adverse effects on environment or human body. For example, antimony trioxide causes weak irritation on skin or mucous membrane and is thus designated as a dangerous drug. Furthermore, the antimony compound as a mineral resource has an increasing risk of unstable supply and an increase in cost because mines are unevenly distributed and the supply-demand balance tends to be tight.
In such circumstance, flame-retardant covering materials not including an antimony compound are demanded. For example, a method of enhancing flame retardancy by adding zinc hydroxy stannate and zinc borate to a vinyl chloride-based polymer is known (see JP-A-H11-080474).